Exactly right. This is the issue.It looks to me like the rubber meets well at the top? Perhaps it also meets well at the bottom, but that is out of shot.. But it looks like the opening window frame has bowed away from the part it's supposed to meet with in the middle
Here are some more pictures, as requested.
From the inside, the bowing can be seen too (along with the air dust) in the middle section of the window.
Please let me know if you require any additional images.
I will await further advice. Thanks again.